Wireless terminals for communication such as terminals are also known as e.g. User Equipments (UE), mobile terminals, wireless terminals and/or mobile stations. Terminals are enabled to communicate wirelessly in a cellular communications network or wireless communication system, sometimes also referred to as a cellular radio system or cellular networks. The communication may be performed e.g. between two terminals, between a terminal and a regular telephone and/or between a terminal and a server via a Radio Access Network (RAN) and possibly one or more core networks, comprised within the cellular communications network.
Terminals may further be referred to as mobile telephones, cellular telephones, laptops, or surf plates with wireless capability, just to mention some further examples. The terminals in the present context may be, for example, portable, pocket-storable, hand-held, computer-comprised, or vehicle-mounted mobile devices, enabled to communicate voice and/or data, via the RAN, with another entity, such as another terminal or a server.
The cellular communications network covers a geographical area which is divided into cell areas, wherein each cell area being served by a base station, e.g. a Radio Base Station (RBS), which sometimes may be referred to as e.g. “eNB”, “eNodeB”, “NodeB”, “B node”, Base Transceiver Station (BTS), or AP (Access Point), depending on the technology and terminology used. The base stations may be of different classes such as e.g. macro eNodeB, home eNodeB or pico base station, based on transmission power and thereby also cell size. A cell is the geographical area where radio coverage is provided by the base station at a base station site. One base station, situated on the base station site, may serve one or several cells. Further, each base station may support one or several communication technologies. The base stations communicate over the air interface operating on radio frequencies with the terminals within range of the base stations. In the context of this disclosure, the expression Downlink (DL) is used for the transmission path from the base station to the mobile station. The expression Uplink (UL) is used for the transmission path in the opposite direction i.e. from the mobile station to the base station.
In 3rd Generation Partnership Project (3GPP) Long Term Evolution (LTE), base stations, which may be referred to as eNodeBs or even eNBs, may be directly connected to one or more core networks.
Universal Mobile Telecommunications System (UMTS) is a third generation mobile communication system, which evolved from the GSM, and is intended to provide improved mobile communication services based on Wideband Code Division Multiple Access (WCDMA) access technology. UMTS Terrestrial Radio Access Network (UTRAN) is essentially a radio access network using wideband code division multiple access for terminals. The 3GPP has undertaken to evolve further the UTRAN and GSM based radio access network technologies.
3GPP LTE radio access standard has been written in order to support high bitrates and low latency both for uplink and downlink traffic. All data transmission is in LTE controlled by the radio base station.
A fundamental of today's wireless communications systems, in particular the wireless radio networks, is the utilization of orthogonal time/frequency resources, e.g. Orthogonal Frequency Division Multiplex (OFDM) in DL, and Single-Carrier Frequency Division Multiple Access (SC-FDMA) in UL. This allows scheduling multiple users at the same time over the operating bandwidth without creating any intra-cell interference, at least in theory, and assuming spatial multiplexing is not used
In order to schedule wireless terminals, whether in DL or in UL, these wireless terminals are informed on which frequency resources they are expected to transmit and receive data, e.g. which Modulation and Coding Scheme (MCS) to use. The method to do so in a LTE system is through the Physical Downlink Control Channel (PDCCH). The PDCCH is broadcasted by a base station every millisecond over a first one, two or three OFDM symbols, out of the 14 OFDM symbols transmitted every millisecond, assuming a normal cyclic prefix. In telecommunications, the term cyclic prefix refers to the prefixing of a symbol with a repetition of the end. PDCCH assignments to wireless terminals are encapsulated into Control Channel Elements (CCE) whose purpose is mainly to simplify the search for the wireless terminal on the PDCCH. PDCCH is a shared resource by both UL and DL wireless terminals and is transmitted in the control region, typically the first one, two or three symbols of a subframe, using 1, 2, 4 or 8 CCEs. A subframe may also be referred to as a Transmission Time Interval (TTI) and represents the shortest transmission interval, in a time domain that can be allocated in an LTE system.
Combined Cell
A combined cell, sometimes also referred to as shared cell, concept refers to configuring two or more adjacent cells with the same Physical Cell Identity (PCI).
An advantage of this concept is that handover between the adjacent cells sharing the same PCI is avoided as these cells will appear as one single cell for the wireless terminal. This results in an increased Signal to Noise and interference Ratio (SINR) at the area that would have been a cell border if non-combined cells were used. Furthermore, if a wireless terminal does not need to perform a handover, it can continuously send and receive data during the time in which it was supposed to do a handover in a non-combined cell deployment. This means that the handover interruption time can be eliminated. In a combined cell deployment, a sector is one out of several transmission and reception points in the combined cell. As an example, the combined cell deployment may be used in a building, using one sector per floor while the whole building is deployed as one single cell as depicted in FIG. 1. FIG. 1 shows a possible configuration of a combined cell deployment with three sectors covering a building, one sector per floor. Different radio units are connected and handled by the same base station, wherein each radio unit provides a respective sector.
As such, a further advantage of the combined cell feature is that the operator can reduce the number of hardware to be deployed, since all the sectors in a combined cell are controlled by the same baseband processing unit of a telecom system. Moreover, similar to Coordinated Multi-Point (CoMP) techniques, the combination of multiple radios offers additional degrees of macro diversity to UL transmissions. The combined cell feature may bring even further advantages if used in combination with Spatial Division Multiplexing (SDM). SDM refers to a technique where spatially-isolated user equipments, i.e. two or more are allocated the same time-frequency radio resources. The OFDM symbols are grouped into Physical Resource Blocks (PRBs). For example, consider one cell with three sectors employing 20 MHz bandwidth with 100 Physical Resource Blocks (PRBs), where all the three sectors share the 100 PRBs if SDM is not supported. Instead, if all the sectors are perfectly isolated, all the three sectors can use 100 PRBs each by means of SDM, thus increasing by 3 times the overall throughput served by the combined cell.
Sector Selection
A critical aspect in a combined cell deployment is sector selection. Sector selection refers to selecting which sector that shall be the wireless terminal's main sector. The main sector is the sector, or transmission point within a cell, to which a wireless terminal has the best quality, e.g. best path gain. A combined cell feature may use a wireless terminal's transmission on a Physical Random Access Channel (PRACH) in order to perform an initial sector selection. This initial selection is then dynamically updated based on e.g. UL Sounding Reference Signals (SRS) to take into account the wireless terminal mobility effects. The wireless terminal mobility effect refers to any change in the wireless terminal's main sector as the wireless terminal moves from one sector to another. In LTE, SRS are typically used for UL Frequency Selective Scheduling (FSS) to assign resource blocks on the basis of instantaneous channel quality. Instead, in a combined cell the purpose of SRS is to estimate the quality of the different sectors in the combined cell and eventually perform sector selection.
Alternatively, other resources for cell sector selection may be exploited to perform sector selection, e.g. by measuring a power received on Physical Uplink Control Channel (PUCCH) Channel Quality Indicator (CQI) resources.
The importance of sector selection stems from the fact that in a combined cell the number of sectors that can be combined is typically smaller than the number of sectors that can be configured, e.g. one possible configuration may be a four sectors combined cell, each served by two transmitter/two receiver (2TX/2RX) antennas with two sectors to be combined. I.e. Two sectors can be combined out of a total of four. Thus, a well-dimensioned and scalable sector selection algorithm is of critical importance to fulfill the promise of a combined cell configuration, e.g. by enhancing the wireless terminal's signal quality reception.
Furthermore, it may be that in some implementation of the combined cell feature, different physical channels may be supported by a different number of sectors, e.g. unlike Physical Uplink Shared Channel (PUSCH), the PUCCH decoding and reception can be performed only in the main sector due to processing complexity limitation. FIG. 2 shows a possible combined cell deployment wherein a base station provides three sectors, and wherein the upper sector is the main sector. As such, possible breakdowns in the sector selection of a certain wireless terminal that moves out of the coverage of the main sector, such as e.g. is moving to another floor in a building as depicted in FIG. 1, may impede the correct detection of Scheduling Requests (SR). SR is used by the wireless terminal to request resources for UL transmissions. If detection of SR is not possible, the wireless terminal will retransmit the SR a large number of times and finally resort to random access to get resources. This will resolve the problem but only after a long delay, resulting in increased load and increased use of the wireless terminal battery.
Commonly, SRS is used to determine the best sector or sectors in a combined cell deployment. In some scenarios, due to the limited SRS capacity, today 192 wireless terminals can be supported using SRS in a combined cell, i.e. total of 192 wireless terminals across all sectors of a combined cell. All other connected wireless terminals in all sectors of the combined cell will use PRACH for sector selection, and the best sector will not be updated as long as the wireless terminal remains connected. Connected means that an RRC connection has been established and the wireless terminal is in state Radio Resource Control (RRC)_Connected. In this state data can be transmitted and received to and from the wireless terminal. In case of low mobility, e.g. in-building deployment, this may not cause any problem, there may be some wireless terminals with sub-optimal sector selection, but the performance should not be severely affected. On the other hand, if wireless terminals move between sectors, which also constitutes a very practical deployment scenario for combined cells, e.g. in highway and railway deployment, the base station would continue to use the initial sector selection for receiving and decoding a wireless terminal's transmission. In general, the base station has a limited processing capability for combining many receive antennas from many sectors. In fact, sometimes it may even not be desirable to combine the signal from all receive antennas from all sectors, e.g. due to diminishing return which means that gains start to flatten out as more sectors/antennas are included, increased complexity, low received signal power in some sectors. As such, it is of significant importance for the base station to know which antennas of which sectors it needs to use for receiving a wireless terminal's transmission.
The sounding capacity may increase in the future. Once this will take place, the sector selection problem will be alleviated. The periodicity of 320 ms may be seen as slow in some scenarios and more frequent sounding may be required, leading to a decrease in the number of wireless terminals that can perform sounding. Although such capacity may be acceptable in today's network load, it would be insufficient when the number of wireless terminals and/or the number of configured sectors per cell further increase. Today up to 4 sectors per combined cell are supported, whereas operators may be interested in configuring many more sectors per combined cell in e.g. highway and railway deployment scenarios. It is also worth noting that not only processing limitation may dictate the limitation on an SRS pool, i.e. the total amount of SRS. For instance, SRS may steal capacity to PUSCH resources, since ideally SRS should be configured such that no overlapping is caused with other PUSCH transmissions in the cell. Hence there might be operators that do not want or cannot activate SRS.
The same arguments are also applicable in case other resources are used to select the best sector or sectors, e.g. PUCCH CQI. Although the number of wireless terminals configurable with PUCCH CQI resources is higher than the number of SRS users, thus providing a higher number of wireless terminals that can be reliably supported by a combined cell configuration, this may still not be compliant with the heavily loaded cellular network foreseen in future deployments.